1. Field of the Invention
The present invention relates to an optical disc recording/reproducing apparatus, and more particularly, to an optical disc defect compensating apparatus and method for improving servo stability when a defect is generated.
2. Description of Related Art
An optical disc recording/reproducing apparatus reproduces information recorded on various kinds of optical discs, such as compact discs (CDs), compact disc-read only memories (CD-ROMs), digital video discs (DVDs), CD-Rs, CD-RWs, DVD-RWs, and DVD-Rs, and records data on the optical discs. The optical disc driving apparatus adapts various servo controls including a mechanical driving control for picking-up RF signals from the discs.
The optical disc recording/reproducing apparatus reproduces a signal according to the intensity of reflected light detected using a non-contact optical head. Since the optical disc recording/reproducing apparatus uses a non-contact optical head, compared to a recording/reproducing apparatus using a contact head, such as a tape cassette deck, quality deterioration can be prevented and reproduction capability is relatively strong.
However, when an optical disc is not handled carefully and/or not stored in a cartridge, for example, the surfaces of the disc can be easily damaged. Typical optical disc defects include scratches or dust buildup on the surface of the optical disc, and interruption, which occurs when zones are omitted during a disc manufacturing process.
Such defects cause reproduced signals to be distorted or omitted, and can even render reproduction of signals impossible, by causing servo signals essential to reproduction to malfunction. In such a case, an optical reproducing apparatus holds a servo signal whenever a specific defect is detected and generates the servo signal again when a defect zone ends.
A conventional defect compensation method will now be described with reference to FIG. 1.
An analog-to-digital converter (ADC) 110 converts an analog tracking error signal generated on a disc into a digital tracking error signal. A high-frequency component compensator 130 compensates for a phase and a gain of a high-frequency component of a tracking error signal. An anti-aliasing filter 160 performs anti-aliasing of the tracking error signal generated by the ADC 110. A low-frequency component compensator 170 compensates for a phase and a gain of the anti-aliased tracking error signal. An adder 140 adds the tracking error signal output from the high-frequency component compensator 130 and the tracking error signal output from the low-frequency component compensator 170 and converts the added signal into a tracking drive signal. A digital-to-analog converter (DAC) 150 converts the tracking drive signal into an analog drive signal and outputs the analog drive signal. When a defect zone where an RF sum signal drops below a specified level is detected, a defect flag is changed to a logic high state, and a contact point 3 of a switch 120 is connected to a contact point 2. Therefore, a hold filter 180 extracts a DC component of the tracking error signal passing through the anti-aliasing filter 160. A register 190 stores the DC component extracted by the hold filter 180. When the defect flag is changed to the logic high state due to defect detection, a servo is held during the defect zone by inputting the DC component stored in the register 190 to the high-frequency component compensator 130. However, at the point of time when the defect flag is changed to the logic high state due to the defect detection, since an error component already exists in the tracking error signal, the servo is unstable. Therefore, since a disc defect is already reflected in the DC component stored in the register 190, if the DC component is used for servo holding, the servo becomes unstable not only during the defect zone but also after the defect zone ends.
FIG. 2 shows waveforms of a defect zone where an RF sum signal drops below a specified level. Referring to FIG. 2, in waveforms at points of time when a tracking servo is turned on again after being held due to sensing of a defect zone, a tracking error (TE) signal and a tracking drive (TRO) signal are severely unstable. That is, since an error component already exists in the TE signal at a point of time when the defect zone is sensed, an output signal is more unstable when passing through the defect zone due to influence of the error component. Therefore, with only a conventional servo holding algorithm, instability in a servo at the start of a defect zone, and increased instability when the defect zone ends, cannot be compensated for. If the TRO signal becomes severely unstable when a servo hold ends, an optical focus may deviate from a current track to an adjacent track.